Different proposals for a seat with a backrest that can be collapsed about a shaft fixed to a seat anchoring element or to a component that is fixed with respect to the seat are known for the purpose of improving user comfort.
More specific proposals seeking that only part of the backrest can adopt different shapes, for example for better supporting the user's lumbar area without the upper part of the backrest changing its orientation or shape are also known.
The proposal described in patent document ES 2323086 is known in this sense, said patent document describing a seat with a collapsible backrest part and a seat part that can shift forward and backward, causing a lordotic-like deformation of the lining of the backrest in the area of the lumbar vertebrae, which is also transmitted to the upper part of the lining in an unwanted manner.
More specifically, according to this proposal the lining of the backrest rests in the area of the lumbar vertebrae on a pivoting body placed between the frame of the backrest and its lining, attached in an articulated manner to the seat part at its lower end. This pivoting body rests, at a certain distance from its articulated attachment with the seat part, against a non-deformable spacing element placed between the pivoting body and the frame of the backrest. As a result, according to the position of the articulated attachment of the pivoting body with the seat part with respect to the vertical of the support point of said pivoting body on the spacing element, the upper part of the pivoting body will be more or less separated from the frame of the backrest, forcing the lining to adopt one shape or another, in this case adopting a shape ranging from significantly bulging forward to slightly bulging backward.
As a drawback, the existence of the spacing element prevents the lining from being able to adopt a shape that is complementary to the shape of the frame of the backrest, since always one of the ends of the spacing body or both ends thereof at the same time will be separated from the mentioned frame of the backrest.
On the other hand, the large separation of the upper part of the pivoting body with respect to the frame of the backrest when the seat part is shifted backward or adopts an intermediate position prevents the lining from being able to be attached to the mentioned frame of the backrest in the middle area thereof. In fact, the deformation of the lining in the area of influence of the pivoting body is such that said deformation is transmitted to the upper part of the lining, which cannot be firmly adhered to the frame of the backrest.
Concerning the articulated attachment between the pivoting body and the seat part, the pivoting body has a pivoting shaft in the part lower which is arranged in the rear area of the seat part, which means that the shaft is shifted during the shifting movement of the seat part to the same extent to which the seat is shifted. Although this can condition obtaining a position of maximum comfort, the articulated attachment between the pivoting body with the seat part is adopted to assure that the separation between the respective linings does not increase, even though their relative position changes, i.e., so that the gap or space separating the lining of the backrest part from the lining of the seat part does not increase.
On the other hand, in the proposal according to ES 2323086 the base frame of the seat on which the seat part shifts has in its front part a sloping area which forces the front area of the seat part to be automatically elevated when the seat part shifts forward. Accordingly, any forward movement of the seat part causes the raising of the front area of the seat part. This can hinder the operation for changing the shape in the lining of the backrest part since the user, in addition to pulling the seat part forward must be careful not to support his/her weight on the seat part in order to not create an obstacle for the lifting of the front part thereof. If the front part is prevented from lifting up, the seat part cannot shift forward and therefore the pivoting shaft around which rotates the pivoting body cannot move. Therefore, the shape of the lining of the backrest part cannot be changed.
A first objective of the invention is an alternative proposal with respect to the one described in patent document ES 2323086.
Another objective of the invention is to also disclose a simpler constructive solution which solves the problems mentioned above and is furthermore compatible with a seat of the type with a frame of the backrest part fixed with respect to a support or base element of the seat, i.e., with a non-reclinable seat.
It is furthermore desirable for the seat to have the maximum number of components in common with that of a seat without the possibility of deformation of the lumbar area of the backrest, and which can more specifically share the lining.